Prior art electro-optical shutters, specifically liquid crystal devices, have had a limited dynamic range (ratio of maximum light transmitted to minimum light transmitted), making them difficult to employ in certain applications. In particular, the surface mode liquid crystal device, which is commonly used in stereoscopic applications, has an on-axis dynamic range measuring about 30:1. Experiments have shown that a much higher dynamic range is required in order to prevent crosstalk between the two channels of a stereoscopic display, i.e., the left and right eye images (see, for example, Lipton, "Factors affecting `ghosting` in time-multiplexed plano-stereoscopic CRT display systems," SPIE Proceedings, Vol. 761, 1987).
Recent experiments indicate that a dynamic range in the neighborhood of 90:1 is required to produce a satisfactory reduction in crosstalk, or, as it is called in the jargon of stereoscopy, ghosting. Such ghosting manifests itself as a double image. Given the present P22 three color phosphor set employed in color CRT's, improvement beyond a dynamic range of 90:1 does not show any reduction in ghosting, since perceived crosstalk derives from two separate physical entities--the incomplete occlusion of the shutter and the phosphor afterglow continuing into adjacent image fields as described in the above-cited article. However, a new tricolor phosphor set is in development which will have shorter afterglow characteristics, in which case even higher dynamic range electro-optical shutters will be required to produce even lower levels of ghosting.
Surface mode devices have become the preferred electro-optical shutter employed in stereoscopic display devices, because of their good transmission, low cost of manufacture, and most important, because they have a brief transition or switching time between transmission and occlusion, and vice versa. Unlike the prior art twisted nematic devices, surface mode devices have a switching time measured in the millisecond range, whereas twisted nematic devices have a transition time measured in the tens of milliseconds. A field sequential electronic stereoscopic display in which the image fields are separated by a vertical blanking interval, typically of a millisecond or less, must employ a selection device with its shutter's transition time of about the same duration as the blanking.
Attempts have been made to improve the dynamic range of electro-optical shutters by employing two surface mode liquid crystal cells in optical series. One approach is called the push-pull device (U.S. Pat. No. 4,792,850, issued Dec. 5, 1989, to Lipton et al.), and another method is employed by Havens ("A Liquid Crystal Video Stereoscope with High Extinction Ratios, a 28% Transmission State, and 100-Microsecond Switching," SPIE Proceedings, Vol. 761, pp. 23-26, 1987). In both devices the shutters are made of two cells in optical series, driven electrically out of phase. The result is an improvement in dynamic range, but with an increased cost of manufacture, higher power consumption, and greater weight.
For a selection device such as CrystalEyes.RTM., which is currently manufactured by StereoGraphics Corporation, it is desirable to use a liquid crystal shutter constructed of a single cell because weight, power consumption, and manufacturing cost will increase with a dual cell approach. CrystalEyes.RTM. wireless eyewear is made up of liquid crystal lenses driven in synchronization with the video field rate through an infrared communications link, and is described in U. S. Pat. No. 4,884,876, issued Dec. 5, 1989 to Lipton, et al., and in pending U.S. Pat. application Ser. No. 387,622, filed Jul. 31, 1989 by Lipton, et al, now U.S. Pat. No. 4,967,268.
StereoGraphics Corporation has been manufacturing the CrystalEyes.RTM. product with an achromatic surface mode liquid crystal device of the type described in above-referenced U.S. Pat. No. 4,884,876. This device has a very high on-axis dynamic range. The mean dynamic range of lenses sampled in manufacture is greater than 150:1, exceeding the requirements of the present P22 phosphor set. If it were required, lenses with a dynamic range of 400:1 or better could be manufactured using this method.
Despite the fact that the achromatic shutter uses a single cell and is light weight, and uses less power than a double cell configuration such as those described above, it requires the application of three laminated layers, rather than the two layers which are usually used with a liquid crystal shutter of the generic type shown in FIG. 8. The two layers referred to are plastic linear polarizer sheets 2 and 4 (shown in FIG. 8), each mounted on either side of liquid crystal cell 6 (shown in FIG. 8). Typically, where cell 6 is a surface mode cell, the director alignment rub axis of the surface mode cell bisects the axes of the polarizers 2 and 4, which are aligned orthogonally.
The achromatic shutter of U.S. Pat. No. 4,884,876 meets the requirements of a high performance selection device, but it is more costly to manufacture than a conventional shutter (of the type shown in FIG. 8) which uses only two laminated layers of polarizer, because the achromatic shutter requires an additional layer. This additional layer is a quarter wave retarder placed between the cell and one of the polarizers. The polarizer which is laminated in juxtaposition to the retarder needs to be adjusted on a lens-by-lens basis in order to produce the highest possible dynamic range. This is a time consuming procedure which has required the building of a special assembly machine. In practice the actual through-put of lenses is restricted by the critical final alignment step.
Therefore, it would be desirable to design an alternative means of producing a high dynamic range shutter without the alignment step with a corresponding reduction in manufacturing cost. Not only would such a shutter reduce the cost of manufacture of the present product, but it would make possible new selection device eyewear for markets requiring a product with a low cost of manufacture. However, until the present invention, it was not known how to design such a shutter.